1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for performing a so-called error diffusion process.
2. Description of Related Art
In image forming apparatuses such as digital copying machines, printers and facsimile machines, a so-called pseudo-halftoning process is performed when multi-level image data indicative of density gradation levels is converted into two-level image data. Particularly, multi-level image data representing a photograph image includes a lot of halftone image data, so that the pseudo-halftoning process is indispensable. One example of the pseudo-halftoning process is an error diffusion process.
In the error diffusion process, an error generated when multi-level image data of an object pixel is converted into two-level image data with the use of a predetermined threshold is diffused to peripheral pixels yet to be subjected to the two-level quantization around the object pixel for modifying multi-level image data of the peripheral pixels, and this error diffusing operation is repeatedly performed. Thus, the density of dots to be outputted from an image forming section is properly controlled, whereby an image represented on a pseudo-halftone basis is outputted. Image forming sections are mostly adapted to form an image on a recording sheet through an electrophotographic process.
An explanation will be given to a case where pixels having image data (white: 0, black: 255) representing multi-level densities on the basis of 256 gradation levels are quantized on a two-level basis through the error diffusion process (two-level error diffusion process);
In the error diffusion process, image data of an object pixel to be compared with a two-level quantization threshold is obtained by adding a cumulative error (the sum of errors distributed to the object pixel from other pixels) to image data of the object pixel indicative of an original multi-level density.
Assuming that the threshold to be employed for the two-level quantization is set at 127 which is a middle value of the 256-level gradation, whether the two-level density of the object pixel is at a white level (0) or at a black level (1) is judged on the basis of the following conditions (A) and (B):    (A) If (Value of object pixel)+(Cumulative error)>127, the two-level density of the object pixel is set at the black level (1); and    (B) If (Value of object pixel)+(Cumulative error)≦127, the two-level density of the object pixel is set at the white level (0).
The error generated when the judgment is made on the basis of the conditions (A) and (B) is distributed to peripheral pixels yet to be subjected to the two-level quantization around the object pixel. When the object pixel satisfies either of the aforesaid conditions (A) and (B), the error is calculated from the following expressions (A1) or (B1) with the black level (1) and the white level (0) in the two-level gradation representation being respectively defined as corresponding to 255 and 0 in the 256-level gradation representation.Error={(Cumulative error)+(Object pixel value)}−255  (A1)Error={(Cumulative error)+(Object pixel value)}−0  (B1)
The error thus calculated is distributed to the peripheral pixels at a predetermined diffusion ratio.
In such an ordinary error diffusion process, dot distribution is at random, resulting in generation of a lot of isolated dots. In the image formation through the electrophotographic process, however, it is impossible to accurately control the size of each of the isolated dots, because dots in an image do not always have a stable size. Where the ordinary error diffusion process is applied to the image formation through the electrophotographic process, gradation representation cannot satisfactorily be stabilized. Therefore, the ordinary error diffusion process is not always suitable for reproduction of a photograph image.
Another example of the halftoning process is an organized dithering process. The organized dithering process includes a dot distribution method (frequency modulation method) in which dots having the same size are distributed at various densities in accordance with gradation levels, and a dot concentration method (amplitude modulation method) in which dots having different diameters according to gradation levels are arranged with the centers thereof being equidistantly spaced from one another. Of these methods, the dot concentration method is more suitable for halftone representation through the electrophotographic process in consideration of the stabilization of the gradation representation.